The invention relates generally to the field of marine electromagnetic surveying of subsurface formations. More specifically, the invention relates to electrical configurations for electromagnetic sensor cables used in such surveying.
Marine electromagnetic geophysical surveying may be used to infer spatial distribution of electrical conductivity of rock formations below the bottom of a body of water such as a lake or ocean. The spatial distribution of conductivity may be used to assist in determining presence of hydrocarbon bearing formations in the subsurface. One type of such electromagnetic geophysical surveying is known as “controlled source” electromagnetic surveying, and generally includes inducing a time varying electromagnetic field in the subsurface formations and measuring one or more parameters related to a response of the subsurface rock formations to the induced electromagnetic field.
Devices for inducing such electromagnetic fields are generally referred to as “sources” or “transmitters” and include, among other devices, spaced apart electrodes or wire loops or coils disposed along or at the end of a cable. The cable may be towed by a vessel in the body of water. Time varying electric current is imparted across the electrodes, loops or coils generally from a power source located on the vessel, to induce a time varying electromagnetic field in the water and subsequently in the subsurface formations. The electrodes, wire loops or coils may be suspended at a selected depth in the water by the use of floatation devices such as buoys, or the source cable itself may be neutrally or otherwise buoyant.
The response of the subsurface formations below the water bottom may be detected by various sensors which may be disposed on long cables or “streamers” towed in the water behind the survey vessel or a different vessel. In some examples, the streamer includes pairs of spaced apart electrodes to detect an electric potential component in the electromagnetic field response.
The direct electromagnetic field strength decreases rapidly with respect to distance from the electromagnetic field source in an electromagnetic (EM) measurement system. The corresponding responsive electromagnetic field modulated by the subsurface rock formations decreases even more rapidly with respect to distance from the source. When electrode pairs are used to detect the electric potential component in the EM field, such pairs need to have a short separation between the elements of the pair when the pair is disposed close to the source in order to not saturate an input amplifier typically associated with the electrode pair. At longer distances (receiver distance from the source, called “offset”) from the source the electrodes in respective pairs may need to be separated by greater distances in order to be able to measure the weaker electric potential component.
Marine streamers are typically assembled from segments each about 75 meters length, and may include a number of such interconnected segments such that the total streamer length may be several kilometers. “Short” electrode pairs can typically be fitted within a standard marine geophysical streamer segment length of 75 m, while longer electrode pair lengths can be many times longer than the length of a typical streamer segment. The spacing requirements for near offset and long offset electrode pairs are thus contradictory with respect to designing a single streamer wiring configuration.
What is needed is a wiring configuration that can be used in a marine sensor streamer having selectable spacing between respective pairs of electrodes.